The functional principle of inductive sensors operating in accordance with the linear variable differential transformer (LVDT) principle is based on coupling between a primary coil and two secondary coils by means of a coupling element. Shifting the coupling element has an influence on the voltages induced in the secondary coils so that drawing conclusions as to the position of the coupling element is possible using the voltages at the secondary coils.
The primary coil here is driven by an excitation signal or primary signal having an excitation frequency. The signal induced may be tapped at both secondary coils. With a symmetrical arrangement of the primary and secondary coils, the secondary signal induced by the primary signal in both secondary coils may be of equal quantity, when there is no coupling element. When there is a coupling element, there is a position thereof where coupling between the primary coil and the two secondary coils is equal such that the two secondary signals are also equal. When shifting the coupling element from said position, the secondary signals change reversely to each other such that a difference signal, which is dependent on the shift, may be achieved. Using so-called amplitude modulation methods, such as, for example, full-wave rectification, separate full-wave rectification including difference formation or synchronous demodulation, the difference signal may be evaluated.
With full-wave rectification, as may exemplarily take place in LVDT positional sensors by the ASM Automation Sensorik Messtechnik GmbH company, the two secondary coils are connected in phase opposition. The demodulated output signal may exhibit a typical V-shape, as is exemplarily illustrated in FIG. 18a. With a shifted coupling element, an amplitude in the output voltage may hint at two positions different from each other. In other words, full-wave rectification induces a problem of lacking unambiguity of the measuring signal. Without further auxiliary measures or further evaluations, it may be impossible to recognize whether the coupling element is on one or the other side of the zero position. Furthermore, the demodulated output signal is originally non-linear and may exhibit an offset which may result in the sensor signal also to comprise a value un-equaling zero in the symmetry position of the coupling element. In accordance with literature, the reason for this is, among other things, flow losses in the LVDT.
A separate full-wave rectification may exemplarily be performed in order to make the measuring signal unambiguous, wherein at first each of the secondary signals is demodulated separately and subsequently a difference is calculated such that the result is an unambiguous characteristic curve, as is exemplarily illustrated in FIG. 18b. Alternatively, full-wave rectification may be combined with phase detection such that a case-by-case analysis for the left and right branches of the V-shaped sensor signal curve is possible, as is exemplarily illustrated in FIG. 18c. 
Both full-wave rectification and separate full-wave rectification and synchronous demodulation, as may exemplarily be performed in accordance with the application document SPRA 946 by the Texas Instruments company, result in additional circuit complexity which may necessitate expensive electronics.
Evaluating the signal amplitudes may also take place using a route mean square-to-direct current (RMS-to-DC) conversion, as is exemplarily described in the Design Note 362 by the Linear Technology company.
Integrated devices which combine several of these functions are, for example, the devices AD598 or AD698 by the Analog Devices company.
When evaluating the sensor signals, digitalization of the signals at the secondary coils may exemplarily be performed by an analog-to-digital converter (ADC).
The amplitude response of LVDT sensors may exhibit strong distortions when there are external magnetic fields or ferromagnetic material, as is exemplarily illustrated in FIGS. 21a to 21d. The result is that sensors operating in accordance with the LVDT principle may be unsuitable in many measuring arrangements where, for example, the sensor is to be accommodated in a metal casing or where there are strong magnetic fields, such as, for example, in electric motors.
Consequently, a device and a method for being able to measure a position of an element unambiguously and independently of external magnetic fields would be desirable.
The object underlying the present invention is providing sensors and methods for operating same which exhibit reduced space requirements and provide a more robust and more reliable detection of positions of movable objects.